A Relay Node (RN) is considered for Long Term Evolution—Advanced (LTE-A) as a tool to improve, e.g., the coverage of high data rates, group mobility, temporary network deployment, the cell-edge throughput and/or to provide coverage in new areas. The RN is wirelessly connected to a wireless communications network via a donor cell (also referred to as a donor enhanced Node B (donor eNB)). The RN may serve as an eNB to one or more User Equipment (UE). To UE that is being served by the RN, the RN may appear identical to an eNB, scheduling uplink (UL) and downlink (DL) transmissions to the UE over an access link, which is between the RN and the UE.
The donor eNB may allocate a portion of its wireless resource (both UL and DL) to the RN so that the RN may service UEs. The wireless resource may comprise of resources in time, frequency, code, or space domain, or a combination thereof. The wireless resource given to the RN may be referred to as a Relay Link (RL) and may be allocated by the donor eNB in a dynamic manner by treating the RN as a UE and assigning transmission opportunities based on requests from the RN, in a persistent or semi-persistent manner by reserving an amount of wireless resource for the RL, or in a combination of both a dynamic and persistent (or semi-persistent manner).
On the DL, the RN receives and buffers DL data intended for UEs being served by the RN from the donor eNB over the RL. The RN may further perform multi-user diversity scheduling to prioritize the transmissions of the data to the intended UEs over the access link, based on channel conditions being experienced by each UE over the access link, as well as based on other criteria, such as Quality of Service (QoS) and fairness. To help ensure the effectiveness of multi-user diversity scheduling, the RN needs to buffer enough DL data for each of a significant number of UEs that it is serving. On the other hand, it is also desirable to minimize the amount of buffer memory required at the RN to reduce cost, power consumption, and physical size of the RN.
On the UL, the RN receives and buffers UL data from UEs that it is serving and then forwards the UL data to the donor eNB over the RL. Again, the RN may desire to use multi-user diversity scheduling. However, in the UL case, the multi-user diversity scheduling occurs over the access link between the UEs and the RN. If a UL buffer for a UE at the RN becomes full, the RN may simply stop scheduling UL transmissions from the UE, resulting in a smaller pool of UEs for the RN to perform multi-user diversity scheduling.
In both the DL and the UL, congestion occurs when the buffers become full. For example, in the DL, a full buffer may lead to loss of data and/or wasted RL wireless resource, while in the UL, a full buffer may negatively impact the RN's ability to fully exploit multi-user diversity scheduling.
Therefore, there is a need for a congestion avoidance mechanism to prevent the donor eNB from sending more DL data to a congested RN in order to prevent loss of data and/or wasting valuable wireless resource as well as to enable the RN's ability to exploit multi-user diversity scheduling of transmissions from the UEs.